Origin of attraction in chalgogen-nitrogen interaction of 1,2,5-chalcogenadiazole dimers.

Intermolecular interaction in the 1,2,5-chalcogenadiazole dimers was studied by ab initio molecular orbital calculations. Estimated CCSD(T) interaction energies for the thia-, selena- and tellura-diazole dimers are -3.14, -5.29, and -12.42 kcal/mol, respectively. The electrostatic and dispersion interactions are the major sources of the attraction in the dimers, although it was claimed that the orbital mixing (charge-transfer interaction) was the most prominent contribution to the stabilization. The induction (induced polarization) interaction also contributes largely to the attraction in the telluradiazole dimer. The large electrostatic and induction interactions are responsible for the strong attraction in the telluradiazole dimer. The short-range (orbital-orbital) interaction (sum of the exchange-repulsion and charge-transfer interactions) is repulsive. The directionality of the interactions increases in order of S < Se < Te. The electrostatic interaction is mainly responsible for the directionality. The strong directionality suggests that the chalcogen-nitrogen interaction plays important roles in controlling the orientation of molecules in those organic crystals. The nature of the chalcogen-nitrogen interaction in the chalcogenadiazole dimers is similar to that of the halogen bond, which is an electrostatically driven noncovalent interaction.